The standards body 3GPP (The 3rd Generation Partnership Project) is in the process of standardizing LTE (Long Term Evolution) as the next-generation W-CDMA (Wideband Code Division Multiple Access) communication standard (for example, see NPL 1 to NPL 3).
In LTE, a wireless communication base station (E-UTRAN NodeB (eNB)) of a network (Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) includes a plurality of communication cells. The wireless communication terminal (User Equipment (UE)) belongs to any one of the communication cells. Hereinafter, the wireless communication base station (eNB) is simply referred to as a “base station”, the communication cell is simply referred to as a “cell”, and the wireless communication terminal (UE) is simply referred to as a “terminal”.
The state of the terminal includes an idle state (RRC_IDLE) in which a radio bearer for transmitting or receiving data to or from the base station is not established and a connected state (RRC_CONNECTED) in which the radio bearer for data communication with the base station is established. When the terminal transmits or receives data to or from the base station, it needs to transition from the idle state to the connected state.
In order to prevent communication failure due to a change in the radio conditions of the component carrier that is being used, the terminal in the connected state monitors the radio conditions and reports the change in the radio conditions to the base station. The report on the change in the radio conditions is transmitted from the terminal to the base station in an RRC (Radio Resource Control) layer or a physical layer. The report in the RRC layer is used to change communication with the cell which is connected to the terminal to communication with another neighboring cell. The report in the physical layer is called a CQI (Channel Quality Indicator) report and is used to monitor the transmission rate in the communication between the terminal and the base station or control transmission power.
Next, the report on the change in the radio conditions which is performed in the RRC layer by the terminal will be described in detail.
The terminal receives the measurement configuration of a reference signal (hereinafter, simply referred to as “measurement configuration”) included in an RRC connection reconfiguration transmitted from the base station. The terminal measures radio conditions (reception power or reception quality) on the basis of the measurement configuration. When an event (for example, a change in event, such as reception power being more than a predetermined threshold value) for transmitting the measurement result occurs, the terminal transmits a measurement report (MR) to the connected base station (hereinafter, referred to as a “Source eNB” or a “movement source base station”).
When the radio conditions are measured at the same frequency as the carrier frequency which is being used, the terminal can perform the measurement while receiving data from the component carrier which is being used. However, during the measurement of the radio conditions at a carrier frequency different from the carrier frequency which is being used, when the terminal does not have capability to monitor a carrier frequency other than the carrier frequency which is being used while monitoring the component carrier which is being used, it needs to temporarily stop the reception of data at the carrier frequency which is being used and monitor the carrier frequency which is desired to be measured. The period for which the reception of data is temporarily stopped is called a “Measurement Gap”. The setting of the measurement gap is notified as control information from the base station to the terminal.
The measurement configuration for causing the terminal to measure reception power or reception quality includes, for example, the following information:
A measurement identifier (MeasID), which is an identifier indicating the measurement configuration;
A measurement object (MeasObject) indicating an object to be measured;
Quantity configuration (QuantityConfig) indicating an operation of filtering the measurement result;
Report configuration (ReportConfig) indicating a trigger standard for transmitting a measurement report (MR) and the format of the measurement report (MR); and
A measurement gap indicating the period for which data for measuring the radio conditions at another frequency or in another system is not transmitted or received.
Among these information items, the measurement identifier (MeasID), the measurement object (MeasObject), and the report configuration (ReportConfig) are associated information items.
FIG. 18 is a diagram illustrating an example of the measurement configuration. As shown in FIG. 18, the measurement configuration is formed by a combination of the measurement object identifier (MeasObjectID) and the report configuration identifier (ReportConfigID). In addition, the measurement object identifier (MeasObjectID) is an identifier indicating the measurement object (MeasObject). The report configuration identifier (ReportConfigID) is an identifier indicating the report configuration (ReportConfig).
FIG. 19 is a diagram illustrating an example of the measurement object (MeasObject). As shown in FIG. 19, the measurement object (MeasObject) includes a downlink carrier frequency (EUTRA-DL-Carrier Freq), a measurement bandwidth, a frequency offset (OffsetFreq), a removal list from a neighboring cell list (CellsToRemoveList), an addition/modification list to neighboring cells (NeighbourCellsToAddModifyList), a removal list from a black listed cell list (BlackListedCellsToRemoveList), and an addition/modification list to black listed cells (BlackListedCellsToAddModifyList).
The report configuration (ReportConfig) includes, for example, the type of trigger of the measurement report (MR), trigger quantity, report quantity, the maximum number of cells to be reported, the period of a report and a report amount. The type of trigger includes, a trigger which is transmitted when an event occurs (event trigger reporting), a trigger which is periodically transmitted (periodic reporting), and a trigger which is periodically transmitted after an event occurs (event trigger periodic reporting). For example, there are five types of events of E-UTRAN, that is, an event in which a serving cell is more than a threshold value, an event in which the serving cell is less than the threshold value, an event in which a neighboring cell is better than the serving cell, an event in which a neighboring cell is more than the threshold value, and an event in which the serving cell is less than threshold value 1 and the neighboring cell is more than threshold value 2.
The contents of the measurement report (MR) varies depending on a target to be measured. For example, the measurement report (MR) has different components when the cell of E-UTRA is measured and when radio access technology (RAT) different from E-UTRA is measured. Next, the measurement report (MR) when the cell of E-UTRA is measured will be described.
FIG. 20 is a diagram illustrating an example of the measurement report (MR). When the terminal measures the cell of E-UTRA, the measurement report (MR) includes components shown in FIG. 20. The head of the measurement report (MR) includes information of a measurement identifier (MeasID), the reference signal received power (RSRP) of a serving cell, and the reference signal received quality (RSRQ) of the serving cell. When the measurement report (MR) includes the measurement result of a neighboring cell, a portion following the head of the measurement report includes information of the neighboring cells (Neighbour cells) which satisfy an event and correspond to the maximum number of reportable cells (MaxReportCells) included in the measurement configuration. The information of the neighboring cell includes a physical cell identity (PCI). In addition, the information of the neighboring cell may optionally include a cell global identity (CGI), a tracking area code, a PLMN identity list (Public Land Mobile Network Identity List). The information of the neighboring cell may optionally include reference signal received power (RSRP) or reference signal received quality (RSRQ). The measurement configuration determines the information of RSRP or RSRQ to be included. When there are a plurality of neighboring cells, the information of the plurality of neighboring cells is included. For example, as shown in FIG. 20, the information of a first neighboring cell (Neighbour cell 1) is followed by the information of the next neighboring cell (Neighbour cell 2).
The value of the component of the measurement report (MR) varies depending on the purpose of measurement. There are three purposes, that is, a search for the best cell, SON (Self Optimizing Network), and the report of CGI. When the purpose is a search for the best cell, the measurement report (MR) has the above-mentioned structure. When the purpose is SON, the measurement report of radio access technology (RAT) different from E-UTRA and one cell to be reported are added as constraint conditions. When the purpose is the report of the CGI, a measurement report including the CGI of a neighboring cell, which is optional, is made. However, when the CGI of the neighboring cell cannot be measured, the restrictions are removed.
The terminal performs measurement indicated by a measurement identifier (MeasID) and transmits the measurement report (MR) to the base station. The base station determines whether to change the cell on the basis of the measurement report (MR). When the cell is changed, the base station determines the cell to be changed. When changing the cell, the base station starts the cell change process.
Next, a report on change in the radio conditions which is performed in the physical layer by the terminal will be described in detail.
The terminal receives a CQI report configuration included in the dedicated physical layer configuration of a dedicated radio resource configuration (Radio Resource Config Dedicated) which is included in an RRC connection reconfiguration transmitted from the base station. The terminal measures a reference signal and creates a CQI report, on the basis of the CQI report configuration. A band, which is a CQI report target, includes a wideband and a sub-band. In the case of the wideband, one CQI report is transmitted in the wideband. In the case of the sub-band, the CQI report is transmitted in each sub-band. In addition, in the case of the sub-band, there are a method of transmitting all sub-band CQI reports indicated by the base station and a method of transmitting the sub-band CQI report selected by the terminal.
Then, the terminal in the connected state checks whether synchronization with the cell is established. When the synchronization is not established, the terminal determines that the radio link is cut and re-establishes RRC connection to another cell. Next, the detailed operation of the terminal will be described.
The physical layer of the terminal monitors the quality of the radio link of the downlink of the cell to which the terminal is connected, and transmits a signal indicating in-sync or out-of-sync to an RRC layer, which is an upper layer of the terminal. When discontinuous reception (DRX) is set, the physical layer of the terminal measures the quality of the radio link at least once for each period of the discontinuous reception. When discontinuous reception is not set, the physical layer of the terminal measures the quality of the radio link for each radio frame. When the quality of the radio link is less than a threshold Qout, the physical layer of the terminal transmits a signal indicating out-of-sync to the RRC layer. On the other hand, when the quality of the radio link is more than a threshold Qin, the physical layer of the terminal transmits a signal indicating in-sync to the RRC layer.
The RRC layer of the terminal determines whether the connection between the terminal and the base station is cut or maintained on the basis of the measurement result of out-of-sync or in-sync transmitted from the physical layer of the terminal. As shown in FIG. 21, when continuously receiving the signal indicating out-of-sync from the physical layer of the terminal a predetermined number of times N310, the RRC layer of the terminal determines that a physical layer problem (PLP) occurs and starts the first operation of the timer. When continuously receiving the signal indicating in-sync from the physical layer of the terminal a predetermined number of times N311 within a predetermined period T310 from the first operation of the timer, the RRC layer of the terminal determines that synchronization is recovered from the PLP and stops the operation of the timer. When the predetermined period T310 has expired, the RRC layer of the terminal determines that RLF (Radio Link Failure) occurs and starts the second operation of the timer. The terminal temporarily stops all radio bearers except for signaling radio bearer 0 (SRB0). The terminal performs cell selection in order to search for a cell for trying to re-establish the RRC connection between the base station and another cell within a predetermined period T311 which is started from the second operation of the timer. When the cell for trying to re-establish the RRC connection is not searched within the predetermined period T311, the terminal transitions to an idle state. When the cell for trying to re-establish the RRC connection is searched within the predetermined period T311, the terminal stops the second operation of the timer and starts the third operation of the timer. The terminal tries to re-establish the RRC connection within a predetermined period T301 from the third operation of the timer. When the re-establishment of the RRC connection is not completed within the predetermined period T301, the terminal transitions to the idle state. When the re-establishment of the RRC connection is completed within the predetermined period T301, the terminal is connected to the cell and stops the operation of the timer.
Examples of the discontinuous reception (DRX) include “shortDRX” and “LongDRX”. The difference between the shortDRX and LongDRX is the length of the period for which data is not received. When only the setting of shortDRX is written in control information from the base station, the terminal performs a shortDRX operation. In the case in which the setting of LongDRX is written in the control information from the base station, when data is not received for a predetermined period of time during the shortDRX operation, the terminal changes the operation mode to Long DRX.